If you only had the one set, I tend to agree, it is possible he has used the few letters he needed. We don't know for sure of course, but I would be strongly showing that it is the set pictured as it could be no other and suggest they re-sell themselves if they are finished with them.

They may have bought the wrong font, you are not obliged to refund in that case.

If they are returned and show as being used, your word against theirs and buyer could embellish scenario to PayPal if case opened.

I suggest staying firm on No refunds given.